Seismic exploration is widely practiced in delineating subsurface layering and location of subsurface structures that may be associated with accumulations of hydrocarbons and other minerals. Initial efforts in seismic exploration involved use of refraction seismographs. Later, reflection procedures were perfected.
More recently continuous wave seismology has been developed wherein a unique non-repeating signal of a few seconds duration is generated by a vibrator at a sending station. A composite signal is detected at a receiving station spaced from the sending station. The composite represents on a time scale the sum of all of the waves traveling from the source directly through near-surface layers, refraction paths and reflection paths from subsurface interfaces.
In continuous wave seismology it is known that increases in time length of the input signal, i.e., the sweep interval, will provide greater volumes of useful signal information. In operation, the signals are recorded in mobile seismic exploration units which generally have somewhat limited data handling capacities. Even though it is known that extending the sweep interval will provide data by which more precise results can be obtained, the volume of signal data that could be made available cannot be readily handled in accordance with conventional present day field techniques.
The present invention is directed towards seismic field operations wherein the sweep interval is increased and the resultant detected signal information is then processed in a unique way to take advantage of the increase in available signal and which may be accomodated in field units to provide greater resolution of the subsurface. By the present invention the signal-to-noise ratio is dramatically increased. At the same time, the complexity of the processing system is substantially reduced. A significant benefit of use of the present invention is elimination of harmonic distortion and enhancement of resolution.
Because of the chirp transform nature of linear frequency modulated sweeps, resulting seismic data can be compressed in real time by a simple multiplication followed by a sequence of low-pass filter-decimation stages. Arbitrarily long sweep times can be accomodated with a modest amount of field hardware. Processing gains in excess of 50 db have been realized.